Electric discharge device and electrode therefor



March 7, 1950 B. JOHNSON 2,499,506

ELECTRIC DISCHARGE DEVICE AND ELECTRODE THEREFOR Filed Sept. 11, 1944 2 Sheets-Sheet 1 DISCHARGE DISC DIAM. 2.26

DISCHARGE CURRENT, AMP. R.M.S. INVENTOR:

LYMAN B. JOHNSON.

BY W HIS 'AT'IORNEY March 7, 1950 1.. B. JOHNSON 2,499,506

ELECTRIC DISCHARGE DEVICE AND ELECTRODE THEREFOR Filed Sept. 11, 1944 2 Sheets-Sheet 2 ,AMF! RMS. PER SQ CM.

2 g 3 3 DISCHARGE CUFIRENRAMP R.M.S PER SQ. CM.

I NVENTOR: LYMAN B. JOHNSON,

DIS CHARGE VOLTAGE BWV M H IS ATTORFEY Patented Mar. 7, 1950.

ELECTRIC DISCHARGE DEVICE AND ELECTRODE THEREFOR Lyman B. Johnson, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Application September 11, 1944, Serial No. 553,531

8 Claims.

This invention relates to electric discharge devices, and is useful in devices producing radiation for various purposes, such as germicidal and therapeutic ultra-violet lamps and tubes, and lamps or tubes used industrially for irradiating or treating various substances and products, or for blueprinting and other photographic purposes, as well as lamps or tubes for more ordinary illumination. Discharge devices for germicidal, therapeutic, irradiating, and photographic purposes are generally of positive column, long gap types in which an arc discharge between widely spaced electrodes is constricted by pressure of the operating atmosphere into a narrow cord, recognizably distinct from the characteristic phenomena closely associated with the electrodes themselves. For an atmosphere, my discharge devices may employ ion izable gas or vapor, or both, such as the rare inert monatomic gases, or vaporizable metal like mercury, for example. Since the coacting electrodes of a discharge device operating on alternating current serve alternately as cathode and as anode, they are generally referred to hereinafter as electrodes" merely, and are distinguished by reference to these alternate functional cycles. The invention is especially adapted for self-heating electrodes, i. e., electrodes that are only heated by the discharge,

and is particularly advantageous in A. C. discharge devices.

This invention involves improvement of that described in my application Serial No. 420,638, filed November 27, 1941, and assigned to the assignee of this application (now U. S. Patent No. 2,363,531, granted November 28, 1944, as well as more distinctive features of novelty.

In most discharge devices manufactured prior to my said application Serial No. 420,638, the electrode(s) functioning as cathode(s) have been activated with materials of low work function, such as thoria' or alkaline earth oxides or mixturesthereof, including barium and strontium oxides. This gives ample electron emission at relatively low electrode temperatures and at low energy consumption for electrode heating, and also avoids electrode disintegration and envelope blackening from vaporization or sputtering of refractory electrode metal. The sputtering here referred to involves a knocking on of particles from an electrode by positive ions of the operating atmosphere that bombard the electrode under the impulsion of a high voltage drop existing adjacent the electrode while it is functioning as cathode, a cathode drop gener- 55 ally much higher than the required ionizing potential of the atmosphere. Sputtering is especially apt to occur during the starting of a discharge.

In attempting to use electrodes without activating oxides, particular difficulties arise from the high work-function of the electrode materials and from cathode hot-spotting, which is a concentration of the discharge and of the heating of the electrode thereby at a'point or area so small that it has to be heated much hotter than adjacent areas in order to give emission corresponding to the discharge current. Field emission due to the cathode drop being virtually nil from an unactivated electrode, the small hot-spot can only yield the required emission thermionically, when heated to an excessive tem perature at whlch the electrode vaporizes rapidly or may even melt; and this requires a high cathode drop, which may produce sputtering while the discharge is starting, if not also during ordinary operation. And since only a small fraction of the energy consumption represented by the cathode drop goes to heat the electrode, while the rest contributes nothing to useful radiant output, the high cathod drop that goes with hotspotting means a relatively poor over-all efficiency.

Because of such difficulties, prior art discharge devices with unactivated electrodes have only been commercially successful in exceptional cases, where electrode disintegration and envelope blackening were unimportant, or where blackening could be limited to envelope areas not required to transmit desired radiation. Another exception is the use of relatively high starting gas pressures, as described in my Patent No. 2,313,646, granted March 9, 1943. Further than this, my application Serial No. 420,638 above-mentioned discloses discharge devices with electrodes of unactivated refractory metal, particularly tungsten, which give a commercially useful life and candle power maintenance, although the efficiencies are not so good as afforded by my present invention.

In theabsence of activating oxides, emission corresponding to the discharge current must be obtained from the electrodes thermionically. Because of their higher work-function, electrodes without oxides must be much hotter than oxide-activated ones, requiring a greater amount of energy for heating them. This would seem to entail a higher cathode drop and a lower efflciency of operation, as is, indeed, the case for the discharge lamps described in my application Serial No. 420,638. Through my present invention, however, I am able to keep unactivated electrodes at the required high temperature by the efficient use of energy inherent in a discharge, without necessity for a large excess (if any) of cathode drop over what is required to ionize the discharge atmosphere, and with correspondingly small loss of efficiency. Indeed, my new discharge devices with such electrodes can equal or surpass the efficiencies of equivalent present-day commercial lamps having activated electrodes. here shown give a higher power'factor, which allows of higher wattage input and greater radiant output without corresponding reduction of useful life.

Discharge devices according to my invention can be made to start easily and operated at good efliciencies that are well maintained over a commercially useful life, without any rapid electrode disintegration or envelope blackenin and without employing activating oxides, or resorting to special measures like high pressures of starting gas. As hereinafter described, my electrodes provide thermionic emission corresponding to the discharge current from arcing portions or members proportioned and presented so as to be covered by the discharge over areas suflicient to yield the required emission at temperatures which avoid substantial, rapid vaporization of the electrode material. In electrodes herein illustrated, the arcing element is mbodied in a solid-centered flattish or disc-like body. For electrodes of tan talum, which are very advantageous, practicable tem eratures are of the order of about 2800" K. to 3000 K., and even somewhat lower or higher, according to the efficiency and the length of commercially useful life desired. The cathode dro is generally low enough to obviate material sputtering or loss of efliciency, and hot-spotting is avoided. To this end, one of my electrodes has an arcing element of compact structure and proportions in which heat that might tend to produce a hot-spot is spread or distributed over a blunt arcing tip or face so that the latter heats and emits quite evenly, affording the required total emission without an excessive temperature.

The tendency of an electrode to overheat locally or to hot-spot is greater when it is functioning as cathode than when it is functioning as anode. A cathode hot-spot tends to form where emission takes place most freely. Whereas heating during the cathode cycles has heretofore been mainly relied on to keep self-heating A. C. electrodes at operatin temperatures, and anode cycle heat has been thrown away, from radiating surfaces, by lead-wire conduction, or by diversion to auxiliary anodes. I maintain the electrode temperature mainly by anode cycle heating, and minimize cathode cycle heat. During the anode cycle, the discharge and the development of heat spread over and cover an arcing face that may be substantially coextensive with the incident arc and bring this face nearly to a temperature affording the required emission. The spontaneous cathode cycle heat which keeps the arcing face fully up to tem erature does not hot-spot the compact heat-distributive arcing portion or member. At the most, the cathode drop wh ch produces this cathode cycle heat need only moderately exceed the minimum that is re quired for ionizing the discharge atmosphere, so that the total cathode drop remains well below the 50 volts (more or less) than refractory metals withstand without disintegration by positive mer- In addition, my new lampscury ion bombardment, or even below the corresponding limit of about 20 volts for electrodes activated with alkaline earth oxides or the like. Unlike the cathode drop, the anodedrop never produces sputtering of an electrode, however high its value may be.

The electrodes hereinafter described involve other important features, which mainly affect the starting of the discharge. In various forms shown, the electrodes are hollowed to lower the starting voltage to a value only moderately exceeding that in ordinary operation, with a low effective cathode drop; the arcing portions are of small enough mass and thermal capacity to heat quickly and provide emission to lower the cathode drop promptly at starting, before appreciable sputtering occurs; additional surfaces are provided to increase the development of heat at starting, and so speed up the heating of the arcing portions. In some forms, parts associated with the arcing portions prevent or minimize escape'of heat during operation, and thus contribute to keep the arcing portions at higher temperature.

Some of the advantages of my invention can be realized in D. C. operation of a discharge device equipped with my electrodes, or in a discharge device equipped with one of my electrodes at one end only, and operated on either A. C. or D. C.

During stabilized operation, pressure of the operating atmosphere constricts the arc and limits the electrode area that it can cover. The higher the pressure, the smaller may be the directly heated arcing surface of a given electrode, and the greater its radiating surface that is not directly heated. Hence large pressure differences may have to be taken account of e. g., an electrode for an operating pressure of many atmospheres may need to be smaller than one for a pressure of one atmosphere, in order that the operating temperature of the arcing portions of the electrodes shall be the same.

Various features and advantages of the invention will become apparent from the description of species and forms of embodiment, and from the drawings. It is also to be understood that features of the devices herein shown and described may be omitted in some cases.

In the drawings, Fig. 1 is a tilted or perspective view of a discharge device or lamp intended to produce therapeutic ultraviolet radiation, and operable in any position from horizontal to vertical, a mid-portion of the lamp envelope being broken out, and a wiring diagram of suitable circuit connections being shown.

Fig. 2 is a large-scale sectional view of an alternative electrode, suitable for other lamps besides that particularly illustrated in Fig. 1, part of the associated current connection (with a fragment of the usual seal glass wrapping" on the inlead) appearing in elevation; Fig. 3 is a similar view illustrating a variant construction of the Fig. 2 electrode, also suitable for other lamps besides that in Fig. 1; and Fig. 4 is a front view of the electrode shown in Fig. 3.

Fig. 5 is a side view of a discharge lamp for i1- luminative purposes in which any of the electrodes shown in Figs. 1, 2, 3, and 4 may be used, and which is intended for vertical operation.

Figs. 6 and 7 comprise curves illustrating characteristics of certain lamps embodying the invenlOIl. Fig. 8 comprises curves more directly useful in designing electrodes according to the invention.

Asshowninl'lahthedisehargedeviceisa lamp having an elongated vitreous, radiationtransmitting discharge envelope I, in the form of a tube of quartz or glass, provided with self-heating solid main electrodes 2, 2 in its opposite ends. and permeable to ultraviolet and visible radiation. A solid auxiliary starting electrode 8 is also shown in one end of the envelope I, closely adjacent the corresponding main electrode 2. The main electrodes 2, 2 are widely spaced to provide for a positive column are discharge between them, long enough to be recognizably distinct from the characteristic phenomena closely associatedv with the electrodes themselves. Besides an atmosphere of starting gas such as one of the inert rare gases like argon, krypton, xenon, etc., the envelope I may contain a vaporizable and ionizable working substance like mercury or other metal. to provide an operating atmosphere at dischargeconstricting pressure during operation. The charge or working substance, represented by a mercury droplet 8, may be more than will vaporize under the heat of the lamp, thus assuring operation with an atmosphere of saturated vapor;

operation on the rated discharge current a mer-- cury pressure is developed which constricts the arc discharge into a narrow cord through the midst of the envelope I. As shown in Fig. 1, the

envelope I is a straight, uniform tube with moldedends somewhat reduced around the electrodes 2, 2, and having flat end walls from which greatly reduced necks 5, 5 proiect around the main inleads 6, 6 and the auxiliary inlead I. These necks 5,5 may embody graded seals, as here shown.

The electrodes 2, 2 and 8 are shown unactivated, consisting merely of bare refractory metal. The auxiliary starting electrode 3 is of an ordi ary form, comprising a simple straight piece of refractory metal wire. while the main oper-- ating electrodes 2, 2 (which may be counterparts of one another) are distinctive. As shown in" Fig. 1, t e ele trodes 2, 2 comprise arci g members. or tips 8, 8 at the inner ends of extension wires 9, '9 welded at ill, ID to the inleads 8, 6, and each arcing member 8 presents toward the other electrade 2 a blunt arcing face from which the discha ge takes on.

Illustrative circuit connections suitablev for the starting and running of my discharge devices are shown in Fig. 1 as including a high-leakagereacta ce transformer ll of semi-auto type with its primary connected across an A. C. power supply circuit i 2 and with its secondaries l3, it connected in series across the main discharge electrodes 2, 2. Through a high current-limiting resistance l5 and a thermal (bimetallic) switch l8, one of the main electrodes 2 and the associated auxiliary starting electrode 8 are connected across the transformer secondaries l3, ll in parallel with the electrodes 2, 2. The heating resistor I! of the thermal switch it is shown con- Q ehoeenastoproduceavoltageoimacrossits serially connected ueondaries l8, M on open circuit, and to give a secondary current of about 3.8 amperes on short-circuit.

Under these conditions, energization of the circuit II will automatically start the auxiliary discharge across the short electrode gap 2, 8 and then the main discharge acrom the electrode gap 2, 2. Thereafter the thermal switch It will disconnect the auxiliary electrode 3 from the secondary circuit, so that the seal around the leads 8, I will not be injured by the D. C. voltage subslsting between them when both are in circuit; and this switch It will remain open at all times when current is on.

As thus far described, the discharge device with its circuit. connections resembles those in my aforesaid application Serial No. 420,638. As regards the electrodes, however. there is a very important difference, which may be summarized by saying that the arcing portions or tips 8, 8 here shown are divorced from much bulkier electrode bodies in which they are incorporated in the discharge devices of that application. In other words, each of the Fig. 1 electrodes 2, 2 is formed by a mere arcing member or tip 8 which need have only enough thickness or extension behind its blunt arcing face to equalize the potential, distribute the heat substantially uniformly throughout said face, and maintain the shape of the member as against warping tendencies due to high temperature. Such a 'flattish body 8 may be broadly characterized as a tablet; for although a nearly flat, round disc is a preferred form, the'part might be triangular, rectangular, or otherwise polygonal rather than circular; need not be truly flat at either nected in one side of the secondary circuit to the main electrodes 2, 2, so as to be heated whenever the arc operates. For a discharge device front or rear; and need only be near enough to uniform in axial dimension to assure: suflicient uniformity of temperature over its front face that is presented to the discharge.

As shown in Fig. 1, each electrode 2 is a mere flat sheet metal disc member 8 mounted perpendicular to the arc stream and to the axis of the envelope l; whereas in Figs. 2, 3, and 4, an arcing tablet or disc 8 is supplemented with an associated rear starting member l8 which affords additional heating surface in starting, assists in heating the member 8 at that time, and also provides a cavity or hollowing to lower the starting voltage, either by its own conformation or in coaction with the member ,8, or both. Besides its functions in starting, the member [8 also reflects back to the member 8 some of the heat radiated from its rear face, thus augmenting its temperature and emission for a given expenditure of energy.

As shown in Fig. 2, the rear'member I8 is also a sheet metal disc, mounted directly behind and coaxial with the disc member 8, and so to speak, parallel to it; and the discs 8, i8 are dished" toward one another in the form of spherical segments, the rear disc I8 to a smaller radius than the front disc 8. The current lead extension or connector 9 that carries the arcing memher 8 in Figs. 1 and 2 is of refractory metal wire so fine and of such resistance that the discharge current heats it to a temperature which may approach that of the member 8 itself, thus providing a heat dam to prevent or minimize cooling by thermal conduction along the wire 8, to say nothing of the inherently low heat conduca tiyity of so fine a wire. By having the length intended to run on a current of 3 amperes at a of Wir 9 b ween he par 8 a d in Fi 2 voltage of 136 volts, the transformer may be sof .75 short and (for most of its length) moderately below the temperature of the member 8, emission from the wire 8 (which varies'exponentially with the temperature) is made so low that'the discharge does not strike or operate on the-wire, either in starting or afterward. As shown in Figs. 1 and 2, the forward end of the wire 9 is bent aside and spot-welded laterally to the rear face of the disc 8, while its rear end is laterally spot-welded at ill to the flattened side of the lead 6. In Fig. 2, the wires extends through a fine central hole in the rear disc member l8, which is attached to the wire 9 by a bent wire I9 welded laterally to the parts. Thus the starting member i8 is electrically connected to the arcing member 8, but is thermally isolated therefrom by the narrowintervening space and the low thermal conductivity of the fine wires 9, l9, as well as by the heat-dam.

The electrode 2 in Figs. 3 and 4 differs from that in Fig. 2 as regards the structure of the rear member [8 and the attachment of the arcing member 8 to it. Instead of being merely dished,

this rear disc member i8 resembles a can or a cake pan, consisting of a ring (either a slice of a tube or an annular sheet metal punching) with a fiat disc 2! abutting against its rear edge or surface and spot-welded thereto. The front member 8 consists of a disc like those in Figs.

' 1, 2, or 3, and corresponds in size approximately to the ring 20, or to its bore. A fine refractory metal wire 22 is spot welded diametrally across the back of the disc 8 and has its ends bent rearward across the outside of the ring 20 and spotwelded thereto, or to the disc 2|, or both. The edge of disc 8 is shown spaced in front of ring 28 a distance substantially or approximately equal to the diameter of the wire 22. The wire 22 is heated by the discharge current to a temperature moderately below that of member 8 for the same purpose as in Fig. 2, and the parts 8, i8 are electrically interconnected but thermally isolated, all essentially as in Fig. 2. The whole Fig. 3 electrode 2 is carried by a current lead extension or connector 9 laterally spot welded to the rear side of disc 2|, and to the inlead 6 at Ill. As shown, this wire 9 is larger than that in Figs. 1 and 2, so that it is not appreciably heated by the discharge current traversing it.

The Fig. 5 lamp is intended to be enclosed in an outer glass jacket fiJled with a gas like nitrogen at a pressure of something like half an atmosphere, like the lamp known commercially as the Type H1 lamp, and is suitable for the same illuminative purposes as that lamp. It should be operated only vertical or upright.

As already mentioned, the arcing portion or member 8 of the form of electrode 2 shown in Figs. 1 to 4' heats very uniformly during ordinary operation or running, and its blunt arcing face does not hot spot. The area of the arcing face 8 and the total exposed surface of the electrode 2 are so correlated with the discharge current that the arcing face 8 yields emission corresponding to the current at a temperature of equilibrium that is practicable for the material in the electrode or its portion 8. Furthermore, the arcing portion or face 8 is brought more or less nearly to this temperature by mere anode cycle heating of the face 8, so nearl that the amount of cathode cycle heat which is spontaneously evolved to keep the arcing" surface 8 fully up to this temperature entails only a moderate electrode drop.

which does not involve any great loss of efliciency of operation. This is explained more fully hereinafter.

8 InFigsJandlthecurvesflandflarethe current-voltage and current-arcing tip temperature characteristics of certain Fig. 1 lampshav- .ing Fig. 3 electrodes of tantalum.* The curves 2! 'are current-arcing tip temperature characteristics for electrodes of these lamps functioning as anodes when the lamps are operated on D. C. currents equivalent in heating effect to the A. C. values here plotted, which are obtained by dividing actual D. C. readings by a conversion factor of 0.45, which is explained hereinafter. Dotted portions of all the curves are extrapolated; The horizontal lines 28 correspond to the lowest voltages on the curves 23, at their high-current ends. The electrodes of the lamps represented by the groups of curves I. II, III, IV, V in Figs. 6 and 7 differ only as regards their discs 3, which have diameters of 1.945, 2.26, 2.49, 2.82, 3.41 mm. are dished to axial depths of about to In these respective diameters, more or less, and are all about V mm. thick, or,more precisely, 0.254 mm. Essentially. the curves 23, 2|, 25, 28 are characteristic of these tantalum discs 8, and would not be materially different if these discs belonged to electrodes of any of the forms shown in Figs. 1, 2, 3, and 4 in either Fig. 1 or Fig. 5 lamps. Other particulars of the lamps and electrodes from which data for the curves were taken are stated hereinafter.

As regards the conversion factor 0.45 above mentioned, it is to be remarked that for a constant anode drop Va and an instantaneous current I, the integral of VaI which represents the heat H received by an electrode during a current and voltage half-cycle on which it functions as anode has the value T nn:

where Inn: is the peak value of I during the cycle. The current being a sine function, 1m equals Irms #2- Visit Irms is the root mean square value of A. C.

the cathode is VaIdc, Idc=0.45 Irms for equal heating on D. C. and A. C.

Unlike the corresponding curve in my aforesaid application Serial No. 420,638, the currentvoltage curves 23 of groups II to V include hori- 55 zontal high-cm'rent portions of constant voltage which the curve 23 in group I is just approaching when the current reaches the practical limit imposed by the melting point of tantalum, 3120 K. Obviously, the vertical distances between asso- 80 ciated curves 25, 24 represent the amounts by which cathode cycle heat on A. C. raises each arcing member 8 above the temperature to which mere anode cycle heating could bring it. As the voltage drop in the positive column of the discharge is practically constant and identical at 76 w i ih excess produces cathode cycle heat to raise the arcing member 8 above the temperature to which anode cycle heating brings it. Accordingly,

I have hereinafter referred to the voltage difference represented by the height of curve 23 above line 28 as excess electrode drop" or "excess voltvoltage drop in the positive column, it is easy to see that the variation of voltage along the curves 28 is really a'variation of the cathode drop; that along the horizontal right-hand portions of the curves 23 of groups II to V, the included cathode drop is constant and of minimum value corresponding to the ionizing potential of the discharge atmosphere, or slightly higher; and that the "excess electrode drop or 'excess voltage above referred to is really an excess of the cathode drop over what is practically required for ionization, which excess (along with said ionizing drop) produces the cathode cycle heat to raise the temperature of the arcing member the amount represented by the vertical difference between the curves and 24.

From comparison amongst the curves of groups I to V, it is seen that for a given current the disc temperature decreases with increasing disc size, while the cathode cycle heat and the excess voltage to bring the disc up to temperature both tend to increase. This shows that the smaller disc is more eflicient for a given current, and corresponds with the increased heat radiation from the larger disc. The larger the disc 8 and the higher the current, however, the more a given excess voltage tends to raise the disc temperature above that which mere anode cycle heating would produce, and the smaller the excess voltage that is required to operate the disc at a given temperature; all of which shows that the efiiciency of cathode cycle heating and of discharge operation increase when discharge current and disc size are correlatively increased.

For a given disc 8, the efficiency is highest in operation on currents corresponding to the horizontal right-hand portion of the curve 23, where the excess electrode drop is zero and the disc temperature is very high. However, it is generally more advantageous to operate further to the left on the curve 23, where the lower temperature allows a longer life at the lower efficiency entailed by some excess voltage. In comparison with operation on currents that involve no excess voltage, the relative efficiency of operation at lower currents may be approximately measured by the ratioi discharge voltage-excess voltage discharge voltage The curves 21, 28, 29 in Fig. 8 are derived from the same discharge lamps as the curves 23, 24, 25, 26 in Figs. 6 and '7, but are more directly useful in designing electrodes to meet various requirements, or for determining the performance of electrodes of sizes different from those represented in Figs. 6 and 7 under various conditions. For the electrode temperature-emissivity linecurve 21, the lamps are operated on D. C. and the facial temperatures of their electrodes functioning as cathodes are read at various current values for which these faces are unformly heated all over. Each current value is divided by the corresponding emissive facial area (that of the front side of the disc 8) and by the square root of two (the ratio of the A. 0. peak to the R. M. S.

value), and the resulting R. M. 8. current intensity is plotted as ordinate against the corresponding temperature. For the discharge current curves 28, each current value for which a curve 28 is desired is divided by the emissive facial areas of the several discs 8; the resulting R. M. S. current intensities are plotted as ordinates against the corresponding areas; and a curve 28 is drawn through all the points obtained for each current value. For the excess-voltage curves 29, a current corresponding to each excess-voltage value for which a curve 29 is desired is found on each of thecurves 23, and is divided by the 'emissive facial area of the corresponding disc 8; the resulting R. M. 5. current intensity is plotted as ordinate against the corresponding area; and a curve 29 is drawn through all the points obtained for each excess-voltage value. As here shown, the ordinates of current intensity are all plotted logarithmically, but on separate upper and lower scales for the curves 21, 28 and the curve 29.

The use of the curves 21, 28, 29 may be illustrated by determining an electrode size for operation on 4 amperes, (1) at 2900 K., or (2) with an excess voltage of' just 4 volts, and (3) by determim'ng the performance of an electrode of 2.65 mm. diameter on a current of 2 amp. To facilitate explanation, the steps are indicated by dash guide-lines and letters.

1) From point A at 2900", the guide line extends up to point B on curve 21 at an emission of 51.5; across to C on the 4-ampere curve 28, at an area of 0.078 sq. cm.; and up to D at the 51.5

,level on the upper ordinate scale, which lies between the 3 and 4 volt curves 29. The electrode should have a diameter of 3.16 mm. and would operate with an excess voltage of about 3%, which means a very good relative efficiency.

(2) From 'point E at an emission of 47 on the 4-volt curve 29, the guide line extends down along the vertical corresponding to an area of 0.085 sq. cm. to point F at an emission of 47 on the 4-ampere curve 28; thence across to point G on curve 21 at the 47 level; and thence down to point H at about 2885 K. The electrode should have a diameter of 3.28 mm. and will operate at about 28B5 K.

(3) An electrode 2.65 mm. in diameter has a facial area of 0.055 sq. cm. From point I at this area, the guide line extends down to point J on the 2 ampere curve 28 at an emission of 45.5; across to point K on curve 21; and down to point L at about 2880 K. On the upper scale, the guide line crossed the 45.5 level at the point M, lying just below the 12.5 volt curve 29. The electrode would operate at about 2880 K. with an excess voltage of about 13.

It will be understood that from similar discharge devices with similar electrodes, but with discs 8 of smaller or larger diameters, additional curves 23, 24, 25, 2'5 can be made, and the curves 21, 28, 29 can be extended both ways. Also, similar curves can be prepared for discharge devices with electrodes made of other -metals than tantalum.

A source of difiiculty in making curv 24, 25, 28, 21 for a set of discharge devices lies n determining the temperature of the member 8 with the optical pyrometer. To do this, the member 8 1 must be looked at aslant, and obliquely through the tube wall I. Variations in the thickness of the glass, in its composition and refractive properties, in the amount of blackening on the wall, or in the angle to the glass or to the member 8 at which the pyrometer is directed aifect the apparent disc color and the temperature reading of the pyrometer. With care, nevertheless, the resulting error can be kept uniform and so small as not to be serious, e. g., not over 5011. Determinations of operating temperatures from the curve 21 show about this same degree of accuracy. Of course data for all curves should be taken from devices having an unsaturated atmosphere of mercury vapor in the range covered, in order to obviate masking of the electrode characteristics by efiects arising from condensation and vaporization of mercury. It will be understood that the curves 2! to 29 as shown in Figs. 6, '7, 8 are illustrative, and have not quite give the shortest discharge path. The transition voltage under which the discharge passes over from glow to positive column is high relative to the voltage for the succeeding table, ordinary operation: e. g., a, Fig. 1 discharge device that ordinarily operates at 136 volts mayrequire a starting voltage of some 300 volts to assure the transition.

With electrodes whose arcing disc members 8.

are associated or supplemented with rear starting members [8, as in Figs. 2, 3, 4, starting is somewhat dii'ferent. The member l8 augments the surface for the initial diffuse glow discharge and also lowers the required starting transition voltvoltage and current and a high cathode drop, resulting in powerful and rapid cathode cycle heating of the member. When a positive column forms and becomes stabilized, the discharge voltage and the cathode drop diminish. As the pressure of the discharge atmosphere builds up, the discharge current falls 01!, the arc stream constricts, and the discharge confines itself to the proximate arcing faces 8, 8 (and perhaps a the accuracy of large scale curves plotted diage, e. g., to some 190 volts as against 300 above rectly from original data on coordinate paper. mentioned. Initially, the glow discharge confines For the convenience of those wishing to use itself to the narrow spaces between the members my invention, I give specific construction data 8, is of Fig. 2 or Fig. 3; but as these members for la p Such as illustrated in 88. 1 and 5: heat somewhat, a glow spreads all over their external surfaces and coacts with the internal glow Envelope tube 1 Fig.1 Fig. 5 in further heating the whole electrode 2. A positive column strikes and becomes stabilized be- Gengfialirrtenaaldialgletenimtween the electrodes 2, 2 and the internal glow usefiuar goes out. As pressure of the operating atmosh l b i] t l ii iii t l li 1arl t e%e% :s i ni g K 5 phere builds up, the discharge current falls off all ubcetlweeregsoigig ig i igmber 6 a and the arc confines itself to the front face and st i: m1 te ii i iie met i i t mus edge of the disc 8. This final transition to the m, e r, g g gg f g g: 8' K arcing face 8 is facilitated by more intense heat- Mei-eut charge (all vaporized), 31111115..-- 0.025 0.200 4 ing of the front member 8 as compared with the Ar Abs-Pressurein "Inning, atmwpheres rear member l8 while the operating pressure gon pressure, mm Rated operating voltage, volts 136 136 builds up toward its final stable value. cumetemwes 3 3 The transitions and the whole starting cycle occur very quickly and smoothly, and substantially without sputtering. In the first place, the Electrodesil, 2, all parts butleadsGoftantalum Fig.1 Fig.2 very small mass and thermal capacity 'Of the disc member 8 allows it to heat up very rapidly Disc-i1? tlliilzlimeter, mm 22 to full operating temperature of the order of some Deg: gfi f f g az i125 1, 3000 K. (more or less), too quickly for any sub- Mounting wi 9, diameter, mils 6 2 g stantial sputtering; and in the second place, the h i ki i ifi 0.254 40 lowered transition voltage obtained by adding the Depth of ishing, mm rear member l8 mainly represents a lowering of i z g ggff edge hind M8 the cathode drop adjacent the electrode, thus re- Wire 19,diemeter,m 6 ducing the sputtering tendency. To assure this, the rear member l8 must not form too large a Fig. 3 electrodes in therapeutic lamps from proportion of the total electrode surface; for which were taken data for the curves ofGroups member 8 must get enough of the glow discharge I, II, III, IV, V in Figs. 6 and 7: urr n to e h ated to a temperature affording Gr. I Gr. II Gr. III Gr. IV Gr. V

1.945 2.20 2.40 2.02 3.41 0.254 0.254 0.254 0.254 0.254 0.25 0.25 0.25 0.25 0.25 0.51 0.51 0.51 0.51 0.51 2.5 2.8 3.0 3.4 4.0 2.0 2.3 2.5 2.0 3.5 2.5 2.8 3.0 3.4 4.0 0.12 0.12 0.12 0.12 0.12 ir 9 d meter m 12 12 12 I: 12 T ii'eta ceendoioteii-eutoisfii: g x M x x In the starting of a discharge between'elecadequate emission. This is all the more importrodes 2, 2 such as shown in Fig. 1, each contant in view of the thermal isolation of the startsisting solely of an arcing tablet or disc 8, a. ing member it from the arcing member 8. The difiuse glow discharge initially spreads over both proportions of parts illustrated in Figs. 2 and faces of each member 8. with a h gh discharge -3 and by specific examples hereinafter meet this requirement.

It is of some importance that the smaller dimension of the space enclosed by the parts 8, I8 is the direct distance between them, so that the space is small, and a large proportion of the dis- I charge energy in this space goes to the arcing member 8 and serves to heat it. If the ring .20 in Fig. 3 is a cylinder longer than its diameter,

narrow peripheral zone adjacent thereto), which 15 too much of the energy may go to and heat this cylinder, and the member 8 may not be sufficiently.

It is unimportant that the cathode drop inside the parts 8, l8 may exceed the sputtering value during starting, because each part 8, I8 regains nearly all that. it loses by sputtering, and very little matter escapes to the envelope wall I to blacken it. The internal glow discharge within the parts 8, l8 (with its essentially innocuous cathode drop) may be regarded as a virtual cathode whose drop to the discharge outside is harmlessly low.

In ordinary operation or running, the relations of the arcing portion or member 8 to the discharge are of great importance. As compared heated with forms like a sphere, a mere rod or wire that is straight, looped, or coiled, or a thin metal sheet in flat, bent, or deeply domed form, the action of a blunt-faced arcing tip 8 such as shown in Figs. 1 to 4 is very different when it is suitably proportioned and presented with respect to the current and to the ,arc stream as determined by the pressure of the operating atmosphere. Since the discharge insists on taking the path of least voltage in the electrode gap, it will only spread over and cover completely an arcing face that is blunt or nearly flat, and (in general) perpendicular to the arc. In the case of a perfectly flat disc of two or three mm. diameter, the arc does not usually extend back on the disc edge more than about of a mm.; while a sphere presenting the same surface area to the discharge tends to hot-spot, in order to produce the required emission from a smaller area, because the voltage drop in the discharge is lowest when the latter extends only to a foremost spot on the spherical surface. For best results, the arcing face should be no larger than the are can heat fairly evenly, without localization of the heat or hot-spotting; at the optimum, it should be just substantially coextensive with the arc stream in spread and surface area.

The arrangement and mass of material behind the blunt arcing face 8 is also of great importance toward securing suificiently uniform facial temperature and emission to provide for the current without overheating anywhere. This calls for compactness of proportions and structure to distribute the heat from the discharge and minimize or compensate for inequalities of heating or of heat loss, e. g., cathode cycle heat tending to produce a hot spot, and losses by conduction and radiation to and from various areas of the part embodying the arcing face 8 and its current lead and support 9 or 22. The heat distribution in the metal interacts with and affects the development of heat over the acting face 8: the more efiectually the temperature is equalized over the face 8, the more uniform the emission and the distribution of discharge current and heat over the face, and vice versa. When the heat is successfully distributed to equalize the temperature, as in an arcing tablet or disc 8 of proper facial area and thickness, the arcing face is heated uniformly on the anode cycles and the cathode cycle heat that spontaneously evolves to keep the face fully up to temperature does not produce a hot spot, or the unevenness of temperature that is an incipient hot spot, on the contrary, the arcing surface has a uniform temperature all over, as determined with the optical pyrometer, and emits uniformly. Indeed, such arcing members can be operated on D. C. without hot-spotting or noticeable unevenness of temperature at either the anode or the cathode, although the cathode drop is higher than on A. C. All this is quite diflerent from the electrode disclosed in myapplication Serial No. 420,638.

The thickness or rearward axial dimension of a blunt-faced arcing member to avoid uneven facial temperature or hot-spotting depends on the facial area; as this is increased, the necessary or allowable thickness likewise increases, as well as the discharge current. It the arcing member is too thin, it heats unevenly or hot-spots because of inadequate thermal conductivity. For example, a disc 2 to 3 mm. in diameter and 0.075 mm. thick operating as cathode hot-spots on a D. C. current of 2 amperes, and shows uneven temperature on an equivalentA. C. current: at a thickness of about 0.11 mm., it is diflicult to g be sure whether a disc of 2% mm. diameter is or is not at an even temperature on a current of 2 amperes D. C.; at a thickness of V or mm., a disc of2 to 3 mm. diameter (i. e., a thickness of about 7-25% of its diameter) shows a uniform facial temperature on currents ranging from 1 to 3 amperes D. C., or 1 /2 to 5 amperes A. C., or even higher for the large discs. Discs of the 2 to 3 /2 mm. sizes that are as thin as 0.075 to 0.11 mm. also tend to warp or curl up toward the discharge when operated on a current of 3 amperes A. 0., though this tendency can be minimized by rearward dishing of the discs as shown in Figs.

2 and 3. Discs about ,4; mm. thick have proved very satisfactory in sizes of the order of 2 to 3 /2 mm. diameter for currents of the order of 2 to 6 amperes A. 0.; their electrical and thermal conductivity make them practicall equipotential in operation, while while their mass and heat capacity are so low that they heat up to operating temperature very quickly in starting. Moreover, only the back face of such a disc (and at most a very narrow edge zone) radiates heat without being directly heated by the discharge and contributing emission thereto.

Beyond a possible greater resistance or immunity to warping over a long life, there is no advantage in increasing the axial dimension or thickness of an arcing member above the minimum required to assure even facial temperature at the rated discharge current, or over the range of current within which the device is expected to operate. On the contrary, a disc that is unnecessarily thick heats up more slowly in starting, because of its greater mass and heat capacity; it entails lower efficiency in operation, because of greater heat losses from its increased radiating surface; and if it is much too thick (e. g., too much above mm. for a disc of 2 to 3 mm. diameter), it shows uneven facial temperature or hot-spots, owing to the heat loss from the excessive peripheral radiating surface. which precludes bringing the arcing face to uniform temperature within the limits of current imposed by vaporization or fusion.

A disc-like arcing member 8 too small in diameter for the discharge current naturally overheats and fuses or vaporizes before reaching a temperature at which the emission suflices for the foil so thin thatits electrical resistance compels the current to spread out in the sheet, do not work; the thin member simply hot-spots and melts, partly because the reduction of electrical conductivity is accompanied by corresponding reduction of thermal conductivity.

Amongst refractory metals that are commercially available for unactivated electrodes, tantalum presents a very favorable combination -01 properties: a work function of about 4.1, a

melting point as high as about 3120' K., and a low vapor-tension and rate of vaporization at temperatures approaching its melting point.

While tungsten melts much higher, at 3640" K.,

its work function is about 4.5, and it vaporizes much more rapidly than tantalum. At temperatures of equal emission for the two .metals. tungsten vaporizes more than four times as fast as tantalum; and so tungsten electrodes necessarily give a shorter life or a lower efficiency than those of tantalum.

Besides equalling or surpassing the efllciency of present-da lamps with activated electrodes, my discharge devices having refractory metal electrodes without activating oxides afford a variety of important advantages overthose with the usual activated electrodes. The electrode construction is simpler and less expensive, special manufacturing steps incident to activated electrodes are altogether avoided, and the operations of exhausting the lamp or the like are materially shortened. The warm-up time in starting the lamps can be made shorter, and changes in the starting voltage do not occur during the useful life. There is no active alkaline material of high vapor pressure to be sputtered or vaporized on to the envelope walls, and hence no devitriflcation or other deterioration of the walls by such material. On the contrary, the only material that can deposit on the walls by sputtering or vaporization is the refractory metal of the electrodes, which is unreactive toward the vitreous material, deposits only on the very ends of an envelope tube, close to the electrodes, and does not migrate along the tube toward its middle after it deposits, as do the ordinary activating materials. Accordingly, the radiant output of the tube is better maintained during its life, both in the luminous range and in the ultraviolet.

A further advantage of my new discharge de-- vices over those with the usual oxide-activated electrodes is a power-factor some 4 to 5% higher. This allows a correspondingly higher wattage input without raising the electrode temperature, giving a like increase in total radiation or lumens from the device.

What I claim as new and desire to .secure by Letters Patent of the United States is:

1. A high pressure electric discharge device comprising a radiation transmitting envelope containing in operation an ionizable gaseous atmosphere at discharge-constricting pressure, and widely spaced, self-heating unactivated electrodes of refractory metal in said envelope, each functioning alternately as anode and cathode of a positive column discharge during A. C. operation of the device, at least one of said electrodes comprising a thermally-emissive arc-supporting disc lying substantially in a plane normal to the direction of the discharge and having a thickness less than the transverse dimension thereof, said thickness and the facial area of said disc being such that the heat from the-discharge at the rated discharge current is distributed substanis tially uniformly throughout the face of said disc and that the said face is brought by anode cycle heating so nearly to a temperature at which its emission corresponds to said current that the cathode cycle heat which keeps said face fully up to said temperature entails only a small excess electrode drop in comparison with the discharge voltage.

2. A high pressure electric discharge device comprising a radiation transmitting envelope containing in operation an ionizable gaseous atmosphere at discharge-constricting pressure, and

- widely spaced, self-heating unactivated electrodes of refractory metal in said envelope, each functioning alternately as anode and cathode of a positive column discharge during A. 0. operation of the device, at least one of said electrodes comprising a thermally emissive arc supporting tantalum disc lying substantially in a plane normal to the direction of the discharge and having a thickness less than the transverse dimension thereof, said thickness and the facial area of said disc being such that the heat from the discharge at the rated discharge current is distributed substantially uniformly throughout the face of said disc and maintains said face at substantially uniform temperature at which the emission therefrom corresponds to said current whereby locally concentrated heating or hotspotting of said face and the large excess electrode drop that this would entail are avoided.

3. A high pressure electric discharge device comprising a radiation transmitting envelope containing in operation an ionizable gaseous atmosphere at discharge-constricting pressure, and widely spaced, self-heating unactivated electrodes of refractory metal in said envelope, each functioning alternately as anode and cathode of a positive column discharge during A. C. operation of the device, at least one of said electrodes comprising a thermally emissive arc supporting tantalum disc lying substantially in a plane normal to the direction of the discharge and having a thickness between approximately mm. and

mm. and a diameter between approximately 2 mm. and 3 /2 mm. for rated currents in the range of about 2 to 7 amperes, said thickness and the facial area of said disc being such that the heat from the discharge at the rated discharge current is distributed substantially uniformly throughout the face of said disc and maintains said face at substantially uniform temperature at which the emission therefrom corresponds to said current whereby locally concentrated heating or hot-spotting of said face and the large excess electrode drop that this would entail are avoided.

4. A high pressure electric discharge device comprising a radiation transmitting envelope containing in operation an ionizable gaseous atmosphere at discharge-constricting pressure, and widely spaced, self-heating unactivated electrodes of refractory metal in said envelope, each functioning alternately as anode and cathode of a positive column discharge during A. C. operation of the device, at least one of said electrodes comprising a thermally-emissive arc-supporting disc lying substantially in a plane normal to the direction of the discharge and having a thickness of about 725% of the diameter of the disc, said thickness and the facial area of said disc being such that the heat from the discharge at the rated discharge current is distributed substantially uniformly throughout the face of said disc and that the said face is brought by anode cycle heating so nearly to a temperature at which its emission corresponds to said current that the cathode cycle heat which keeps said face fully up to said temperature entails only a small excess electrode drop in comparison with the discharge voltage.

5. A high pressure electric discharge device comprising a radiation-transmitting envelope containing in operation an ionizable gaseous atmosphere at discharge-constricting pressure, and widely spaced self-heating .unactivated electrodes of refractory metal in said envelope, each functioning alternately as anode and cathode of a positive column discharge during A. C. operation of the device, at least one of said electrodes comprising a thermally-emissive arc-supporting disc lying substantially in a plane normal to the. di-

rection of the discharge and having a thickness less than the transverse dimension thereof, said thickness and the facial area of said disc being such that the heat from the discharge at the rated discharge current is distributed substantially uniformly throughout the face of said disc and that the said face is brought by anode cycle heating so nearly to a temperature at which its emission corresponds to said current that the cathode cycle heat which keeps said face fully up to said temperature entails only a small excess electrode drop in comparison with the discharge voltage, said one electrode also comprising a disc-like rear member essentially parallel to and electrically connected to said arc-supporting disc but separated therefrom by a narrow space so that during starting said rear member affords additional starting surface and coacts with said aresupporting disc in heating the same as well as in lowering-the starting voltage of the device.

6. A high pressure electric discharge device comprising a radiation-transmitting envelope containing in operation an ionizable gaseous atmosphere at discharge-constricting pressure, and widely spaced, self-heating unactivated electrodes of refractory metal in said envelope, each functioning alternately as anode and cathode of a positive column discharge during A. C. operation of the device, at least one of said electrodes comprising a thermally-emissive arc-supporting disc lying substantially in a plane normal to the direction of the discharge and having a thickness less than the transverse dimension thereof, said thickness and the facial area of said disc being such-that the heat from the discharge at the rated discharge current is distributed substantially uniformly throughout the face of said disc and that the said face is brought by anode cycle heating so nearly to a temperature at which-its emission corresponds to said current that the cathode cycle heat which keeps said face fully up to said temperature entails only a small excess electrode drop in comparison with the discharge voltage. said one electrode also comprising a disc-like rear member essentially parallel to and slight hollowed toward and electrically connected from by a narrow space so that during starting said rear member affords additional starting surface and coacts with said arc-supporting disc in heating the same as well as in lowering the starting voltage of the device.

7. A high pressure electric discharge device comprising a radiatiomtransmitting envelope containing in operation anionizable gaseous atmosphere at discharge-constricting pressure, and widely spaced, self-heating unactivated electrodes of refractory metal in said envelope, each functioning alternately as anode and cathode of a positive column discharge during A. C. operation of the device, at least one of said electrodes comprising a thermally-emissive slightly rearwarddished front arc-supporting disc lying substantially in a plane normal to the direction of the discharge and throughout which the heat from the discharge at the rated discharge current distributes itself substantially uniformly, and a rear starting disc member essentially parallel to and slightly hollowed toward said arc-supporting disc and electrically connected thereto but spaced therefrom.

8. A high pressure electric discharge device comprising a radiation--transmitting envelope containing in operation an ionizable gaseous atmosphere at discharge-constricting pressure, and widely spaced, self-heating unactivated electrodes of refractory metal in said envelope, each functioning alternately as anode and cathode of a positive column discharge during A C. operation of the device, at least one of said electrodes comprising a thermally-emissive front arc-supporting disc lying substantially in a plane normal to the direction of the discharge, and a rear starting disc member essentially parallel to and slightly hollowed toward said arc-supporting disc and electrically connected thereto, said rear member comprising an annulus spaced slightly behind said arc-supporting-disc and having a disc closing the opening of said annulus.

LYMAN B. JOHNSON.

REFERENCES CITED The following references are of record in the file of this patent:'

UNITED STATES PATENTS Number Name Date 2,008,066 Ende Ju1y16, 1935 2,018,957 Hyde Oct. 29, 1935 2,060,610 Cox Nov. 10, 1936 2,177,690 Davies Oct. 31, 1939 2,249,094 Seitz July 15, 1941 2,249,672 Spanner July 15, 1941 2,267,318 Aicher ..l Dec. 23, 1941 2, 2,313 Gustin Nov. 17, 1942 2,310,983 Miller, Feb. 116, 1943 2,342,806 Hofmann Feb. 29, 1944 FOREIGN PATENTS Number Country Date 695,645 Germany Aug. 29, 1940 to said arc-mpporting disc but separated there- Certificate of Correction Patent No. 2,499,506 March 7, 1950 LYMAN B. JOHNSON It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 12, line 4, for the Word table read stable; column 14, line 35, strike out while, first occurrence;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 29th day of August, A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

